The possibility of a radiologic terrorist attack, accidental radiation exposures, or radiation accident has led to a critical need to identify agents with the capacity of radioprotection or mitigation of potentially lethal total or partial body radiation exposures. Three classic acute radiation syndromes exist: bone marrow (hematopoietic) syndrome, gastrointestinal (GI) syndrome, and cardiovascular or CNS syndrome. GI syndrome generally occurs in the dose range of 6-10Gy in humans (threshold) and is lethal within two weeks. In GI syndrome, the GI tract is irreparably damaged with massive necrosis and apoptosis of enterocytes. Denudation of the intestinal epithelia leads to dehydration and electrolyte imbalance via a secretory diarrhea. Additionally, the transmigration of bacteria through the damaged intestinal mucosal barrier in combination with bone marrow suppression results in a high risk of infection and sepsis. Death of both differentiated cells and stem cells in the marrow and gut are implicated in the pathogenesis of GI syndrome. Identification of agents that can protect intestinal cells and bone marrow from radiation injury (protectors) and agents that can mitigate the effects of potentially lethal exposures (mitigators) are needed. In order to exhibit maximal efficacy for the context of potentially lethal total body exposures that results in GI syndrome, any identified agents should be effective in both tissue compartments (gut and marrow). Traditional compound screening for agents effective in preventing GI syndrome requires a substantial investment in animal models in which an LD50/10 is calculated (multiple doses of radiation, multiple doses of candidate reagent, multiple times of exposure in relation to radiation). These studies are costly and time consuming. The ability to screen compounds in vitro in a fashion that recapitulates the in vivo condition would significantly facilitate identification of agents for in vivo study. Our laboratory has recently developed in vitro methods that allow testing of intestinal organoids in a fashion that recapitulates in vivo results obtained with known radiation protectors and mitigators (presented at Radiation Research Society 2015). This project will leverage that expertise to identify candidate agents for study.